1. Field of the Invention
This invention relates to a method, system and device wherein, for example, anti-duplication or other additional information is superimposed on a video signal and transmitted, the additional information superimposed on the video signal is detected on a receiving side, for example by a recorder, and anti-duplication control or other processing is performed.
2. Description of Related Art
Devices for recording digital information, such as digital VTR and MD (mini-disc) record players, are in widespread use, and DVD (digital video disc or digital versatile disk) devices with recording functions have also appeared.
In these digital information recorders, various additional information signals can be recorded together with the main video and audio signals or computer data, etc. In this case, the additional information signal is a digital signal recorded on an area distinct from digital information signal areas, such as for example the headers added to every block of data or TOC (Table of Contents) area.
In the case of a system where additional information is transmitted by superimposing on a conventional main video signal, therefore, the additional information signal is not superimposed directly on digital information signals such as headers, but is recorded on an indirect area.
The additional information signal may therefore easily be eliminated by filtering or tampering, so that necessary additional information signals can no longer be detected by recording and playback devices. In particular, when control information and copyright information are added as additional information signals to prevent illegal duplication, their original purpose cannot be achieved due to the loss of the additional information signal.
Moreover, if the additional information signal is added to an indirect area, only the main information signal will be obtained when the digital information signal is converted to an analog signal and the additional information signal will be lost. Even if an anti-duplication control signal is added as an additional information signal to prevent illegal duplication, this strategy is no longer effective when the signal is converted to an analog signal.
To solve this problem of the disappearance of the additional information signal when the signal is converted to an analog signal, the inventors already proposed a method wherein an additional information signal such as an anti-duplication signal is spectrally spread, and the spectrally spread additional information signal is superimposed on a video signal during digital or analog recording (U.S. patent application No. 08/75,510).
In this method, spectral spreading is performed by, for example, generating a code of a PN (Pseudorandom Noise) sequence (hereafter, referred to as a PN code) at a sufficiently high rate and applying it to the additional information signal. An additional information signal such as a narrow-band, high level anti-duplication control signal is thereby converted to a wideband, very low level signal which has no effect on the video signal. The additional information signal which has been spectrally spread in this way, i.e. the spectrally spread signal, is then superimposed on an analog video signal and recorded on a recording medium. The video signal recorded on the recording medium may be either analog or digital.
In this method, the additional information signal, such as an anti-duplication control signal, is superimposed with the same timing and frequency as the video signal. It is therefore difficult for a person who wishes to perform illegal duplication to remove the superimposed anti-duplication control signal from the video signal. However, the additional information signal such as a superimposed anti-duplication control signal can still be detected and used by performing despreading.
In this case, the additional information which comprises the spectrally spread signal is not removed from the video signal by detection. Therefore, the additional information must be superimposed on the video signal at a very low level which does not affect the image reproduced from the video signal, but although it is at a very low level, it must be a sufficient level for the additional information to be detected.
The spectrally spread additional information is superimposed on the video signal as a wide-band, low-level signal, and to avoid deterioration of the video signal, it should be superimposed at a low level which is no greater than the S/N ratio of the video signal.
Hence, the spectrally spread additional information is superimposed on the video signal at a low level less than the S/N ratio of the video signal. For example, to enable detection of the additional information of the spectrally spread signal superimposed on the video signal in the recording device, the number of PN codes (PN code length) required to spectrally spread one bit of the additional information must be sufficiently large. The PN code length per bit of this additional information signal may be expressed as a spreading gain (spreading factor) which is a ratio (T/TC) of a time width T per bit of additional information to a time width TC of one PN code (one chip). This spreading gain is determined according to the S/N ratio of the information signal on which it is attempted to superimpose additional information, which in this case is the S/N ratio of the video signal.
For example, when the S/N ratio of the video signal on which the additional information is to be superimposed is 50 dB, the additional information superimposed on the spectrally expanded video signal must be superimposed at a lower level than 50 dB which is the S/N ratio of the video signal. At the same time, to detect the additional information superimposed on the video signal, the S/N ratio must be sufficient for demodulating the additional information after spectral spreading. If this S/N ratio is 10 dB, a spreading gain of 60 dB=(50 dB for S/N ratio of video signal)+(10 dB for SIN required for detection) is required. In this case, the PN code length per bit of additional information becomes one million.
The additional information should be superimposed on the video signal at as high a level as possible, but if the superimposition level of the additional information, which is the spectrally spread signal, is even a little too high, the superimposed additional information will cause visual interference and tend to stand out.